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CLAYS

145

most of the acid being then evaporated on a sand-bath and the remainder
taken up in water ; the clear liquid is decanted off and the insoluble residue
digested with hydrochloric acid and then diluted with water and heated,
the silica being filtered off and washed. Without being ignited, it is then
digested with 10% sodium carbonate solution, the further procedure being
as with limestone (q.v., Complete Analysis, 6, b).

4. Alumina, Oxides of Iron and Manganese.—In the filtrate from
the total silica the iron is oxidised with nitric acid and precipitated along
with the alumina, as already described (see Limestone, Complete Analysis, 7).

If manganese is present in sensible amount, the filtrate from the silica
is neutralised with sodium carbonate, treated with neutral concentrated
sodium or ammonium acetate solution, diluted with water and heated to
boiling for a minute ; the liquid is then filtered and the precipitate washed
by decantation with boiling water containing a little sodium or ammonium
acetate. The precipitate is next redissolved in hydrochloric acid and the
aluminium arid iron precipitated with ammonia in the ordinary way. The
filtrate from the precipitation of the basic acetates is acidified slightly with
acetic acid and the manganese precipitated as hydrated peroxide by addition
of bromine water to give a brown coloration and then of excess of ammonia,
the liquid being heated to boiling until the precipitate separates from the
liquid in flocks. After settling, the precipitate is filtered, washed with
boiling water, dried and ignited in a blowpipe flame to transform it into
Mn3O4, which is weighed. Mn304 X 0-93007 = MnO.

5.  Lime and Magnesia.—These are determined in the filtrate from
the preceding determination, operating as with limestone (q.v., Complete
Analysis, 8).

6.  Alkalies.—A fresh portion of the substance (2-5 grams) is heated
in a platinum crucible on a water-bath with hydrofluoric acid in presence
of a little sulphuric acid, the excess of acid being evaporated when all the
silica has been expelled.   The residue is then taken up in hydrochloric acid
and hot water, and the sulphuric acid, aluminium, iron and magnesium
precipitated with excess of barium hydroxide at boiling temperature.

From the filtrate the excess of baryta and the lime are eliminated by
digestion in the hot with ammonium carbonate and filtration, the filtrate
being evaporated to dryness, the residue heated to expel ammonium salts,
the residue dissolved in water and the solution filtered and evaporated to
dryness in a tared platinum dish. This residue consists of the chlorides
of any sodium and potassium present.

If required, the separate determination of the two alkali metals may
be carried out by the indirect volumetric method (titration of the total
chlorine in a known weight of the chlorides ; see Fertilisers, p. 135) or by
the gravimetric method (precipitation of the potassium as platinichloride),
the procedure being as described with fertilisers (p. 124).

7. Sulphates, Sulphides.—These are determined as with limestone
(q.v., Complete Analysis, 9 and 10).

*
* *

The deductions to be drawn from the composition of a clay, in so far as these
are of interest in the manufacture of cement, are based (i) on the proportions
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